The present invention relates to a band type strapping machine. More particularly, it relates to a band feeding and pull-tightening means in the band type strapping machine, in which operations for feeding and pulling a plastic band can be performed with a simple structure.
There have been known band type strapping machines which have a band guide on a table on which a package to be packed is placed and are adapted to feed a plastic band around the package, the band then being pulled back to be tightened on the package. The conventional band type strapping machine are generally classified into two groups. The strapping machines belonging to the first group are ones such that, for instance, as shown in Japanese Examined Utility Model publication No. 56082/1980, a feeding roller and a tightening roller are placed at a juxtaposition and a driving roller is disposed above each of the rollers so that the driving rollers are selectively brought into contact with the feeding and tightening rollers, whereby a band-feeding or band-tightening operation is performed. In this case, a gearing device is interposed between the feeding roller and the tightening roller so that the former is driven to feed the band, while the later is driven to return and tighten the band on the package.
The strapping machines belonging to the second group are so constructed that a single driving roller is driven by a reversible electric motor and the roller is brought into contact with a driven roller to feeding or returning a band.
The strapping machines of the first group have disadvantages that they push up cost for manufacturing because the gearing device is interposed between the feeding and tightening rollers and that they generate large noise during operations.
The strapping machines belonging to the second group have a problem of cost because the expensive reversible motor has to be used.
Further, in the conventional band type strapping machines, there has been known means for adjusting tension of the band when the driving roller is forcibly brought into contact with the return roller to pull back and tighten the band. Such means is published in, for instance, Japanese Examined Utility Model publication No. 23280/1965. The tension adjusting means is provided with an inclinable lever, a pawl and a receiving piece in which the pawl and the receiving piece are fitted to the inclinable lever. When the inclinable lever is inclined, a band is gripped by the pawl and the receiving piece so that the band is forcibly pulled in the direction of returning the band by the action of a spring provided in the inclinable lever. When the inclinable lever is inclined in the opposite direction, the pawl is disengaged from the receiving piece to release the band.
Thus, in the tension adjuster which performs feeding of the band by causing inclination of the lever in one direction to release the band, and strongly returning the band by causing inclination of the lever in the opposite direction, movement of inclination of the lever is actuated by detecting tension of the band with use of a band tension sensing means by which a solenoid is energized or deenergized.
The conventional machine, however, has a complicated structure such that provision of the solenoid device for actuating the inclinable lever, the band tension sensing means for actuating the solenoid device, a spring for strongly returning the band, and so fourth is needed. Further, accuracy of timing of actuating solenoid device is not satisfactory.